J. Am. Chem. SOC. 1995,117, 7535-7543
7535
Reaction of Phosphaethene with Hydrogen Isocyanide: [2+ 13 versus [2+2] Cycloaddition Minh Tho Nguyen,* Annik Van Keer, Kristine Pierloot, and Luc G. Vanquickenborne Contribution from the Department of Chemistry, University of kuven, Celestijnenlaan 200F, B-3001-kuven, Belgium Received February 21, 1995@
Abstract: The model reactions of hydrogen isocyanide plus phosphaethene and its protonated form have been studied by ab initio MO methods. The energy potential surfaces have been explored using MP2/6-31G(d,p) calculations while relative energies between stationary points have been estimated using QCISD(T)/6-3 lG(d,p) and CISDQ/6311G(d,p) wave functions in conjunction with zero-point corrections. The HNEC CH2=PH system is also considered at a higher level by means of the CASFT2/ANO method. With regard to the [2+1] cycloaddition giving phosphiranimine ( l ) the , initial C-attack with an energy barrier of 25 kcal/mol is slightly favored over the P-attack. Two transition structures for [2+2] cycloaddition have also been found, but this approach is much less favored than the [2+1] approach with a difference of at least 30 kcal/mol in the barrier heights. Two four-membered carbene rings resulting from the [2+2] cycloadditions exist; the most stable of the two adducts lies 29 kcal/mol above 1 and is also an intermediate in the rearrangement of 1 to phosphaziridine 2 having an exocyclic C-C double bond (6 kcal/mol above 1). The isomer with an exocyclic C=P bond (3) lies 16 kcal/mol above 1. Overall our results suggest that the [2+l] approach is the dominating mechanism of the H N W H2C=PH reaction. The participation of the [2+2] cycloaddition in the transformation can be ruled out. The [2+1] cycloaddition giving 1 is also the rate-determining step. Owing to the fact that the rearrangement 1 2 requires a smaller activation energy and that the energy difference between 1 and 2 is small (6 kcaumol), 2 could be formed as a minor product. Thus, the reaction differs fundamentally from that involving the isoelectronic HNEC H2C=SiH2 system where a fourmembered carbene has been found to have a high stability. Protonating the phosphorus atom substantially stabilizes the cyclic structures. The four-membered carbene is stabilized by about 9 kcal/mol relative to the neutral structure l a . Nevertheless, the only thermodynamicallyfavored process in the HNW H2C=PH2+ system is a C-nucleophilic addition giving an open nitrilium cation which is by far the most stable protonated species.
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Introduction Isocyanides are versatile building blocks in organic synthesis. Owing to the presence of a monocoordinated carbon atom, these compounds are well suited as 1,l-dipolar partners in diverse addition reactions whose primary products are expected to undergo further rearrangements or ring expansions. Although isocyanideshave been known to add to dipolarophilescontaining second-row atoms such as disilenes (R2Si=SiR2),' diphosphenes (RP=PR),233 and silenes (R2C=SiR2),4 yielding different heavier analogues of methylenecyclopropanes, their reactivity toward other doubly-bonded compounds is not yet known. Brook and co-workers5 showed that an isocyanide (I) undergoes a formal [2+1] cycloaddition to a stable silene (11) yielding an unstable siliranimine (111)which rapidly rearranges to form the isomeric silaziridine IV (eq 1). The isomerization HI IV is intriguing in many aspects, in particular as concems the relative stability of both products as well as the reaction mechanism.
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Abstract published in Advance ACS Abstracts, June 15, 1995.
(1) Yockelson, H. B.; Millevolte, A. J.; Haller, K. J.; West, R.J . Chem. Soc., Chem. Commun. 1987, 1605.
(2) Baudler, A. M.; Simon, J. Chem. Ber. 1987, 120, 421. (3) Lentz, D.; Marshall, R. 2. Anorg. Allg. Chem. 1992, 617, 5 3 . (4) Brook, A. G.; Kong, Y. K.; Saxena, A. K.; Sawyer, J. F. Organometallics 1988, 7, 2245.
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In a recent theoretical study6 on the model H N W H2C=SiH2 system, using ab initio molecular orbital calculations, we have shown that the unimolecular rearrangement I11 IV occurs preferentially in two distinct steps involving a fourmembered cyclic carbene (V) as the intermediate (eq 2). Surprisingly, the relative stabilities of V and I11 (R = H) are of comparable magnitude and the formation of V constitutes the rate-determining step of the entire transformation.
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III, R=H
V
N.R=H
From a geometrical viewpoint, V could formally be regarded as a product of a [2+2] cycloaddition of HN- plus HzC=SiH2. Although the relevant [2+2] transition structure could not be located in our earlier study, the cyclic carbene establishes a genuine bridge between both [2+1] and [2+2] approaches. These theoretical results raise the question as to whether the existence of a stabilized carbene intermediate is merely a particularity of the silicon system or rather represents a more general phenomenon. In an attempt to tackle this question, we have considered the phosphorus analogue HNEC CH2=PH. In this work, particular attention has been paid to both [2+2]
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( 5 ) Brook, A. G.; Saxena, A. K.; Sawyer, J. F. Organometallics 1989, 8, 850. (6) Nguyen, M. T.: Vansweevelt, H.; De Neef, A,; Vanquickenbome, L. G. J. Org. Chem. 1994, 59, 8015.
0002-7863/95/1517-7535$09.00/00 1995 American Chemical Society
7536 J. Am. Chem. SOC.,Vol. 117, No. 28, 1995 and [2+ I] cycloaddition pathways, the interconversion between cycloadducts, and the formation of other rearranged adducts (eq 3).
For the purpose of comparison with H2C-SiH2, the reactions involving the protonated phosphaethene H N I C H2C=PH2+ have also been examined. As far as we are aware, no experimental study on these reactions has been reported even though the reactions of isocyanides with diphosphenes ( W P R ) are known e~perimentally.~,~
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Calculations and Results Ab initio molecular orbital calculations were carried out making use of local versions of the Gaussian 92 and Molcas-3 sets of program^.^,^ Stationary points on the [ C 2 W ] and [CzHsNP]+ surfaces were initially determined at the HartreeFock (HF) level with the 6-31G(d,p) basis set and characterized by harmonic vibrational analysis at this level. Geometries of the relevant points were fully reoptimized using second-order Mgller-Plesset perhxbation theory (MP2/6-3 lG(d,p)). Relative energies were then computed using the quadratic configuration interaction method QCISD(T)/6-31G(d,p) as well as the configuration interaction method CISDQ with a larger 6-3 1lG(d,p) basis set and MP2 optimized geometries. In order to assess further the influence of electron correlation on relative energies, we have also constructed multireference wave functions using the CASSCF-CASPT2 methodg3l0for all structures involving the HNEC CH2=PH system. In these calculations, generally contracted atomic natural orbital type basis sets (ANO) were used. The starting primitive sets of functions (13slOp4d) for P, (10s6p3d) for C and N, and (7s3p) for H were contracted to the following final basis sets: [5s4p2d] for P, [4s3pld] for C and N, and [3slp] for H." Only the pure spherical harmonic components of the d-functions were used, yielding a total of 105 contracted AN0 functions. Each of the CASPT2 calculations is performed in two steps: the CASSCF wave function determined in the f i s t step is used to the second step as the reference function for a second-order perturbation calculation. The CASPT2 method computes the first-order wave function and the second-order energy in the full space of configurations generated by the basis set. The zero-order Hamiltonian is constructed from a Fock-type one-electron operator that is reduced to the second-order MZller-Plesset operator in a singlereference closed-shell case. The "nondiagonal" approach, which consists of taking the full Fock matrix (with all nondiagonal elements), is used in constructing the zero-order Hamiltonian. In the CASSCF calculations, 10 electrons were correlated in
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(7) Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. G.; Gompert, R.; Andres, J. L.; Raghavachari, R.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. GAUSSIAN 92; Gaussian Inc.: Pittsburgh, PA, 1992. (8) Andersson, K.; Blomberg, M. R. 4.; Fiihscher, M. P.; Karlstrom, G.;Kello, V.; Lindl, R.; Malmqvist, P. A.; Noga, J.; Olsen, J.; Roos, B. 0.;Sadlej, A. J.; Siegbahn, P. E. M.; Urban, M.; Widmark, P. 0.MOLCASVERSION 3; Lund, 1994. (9) Andersson, K.; Malmqvist, P. A,; Roos, B. 0.; Sadlej, A. J.; Wollinski, K. J . Phys. Chem. 1990, 9$, 5483. (10) Andersson, K..; Malmqvist, P. A.; Roos, B. 0. J . Chem. Phys. 1992, 96, 1218. (1 1) Pierloot, K.; Dumez, B.; Widmark, P. 0.;Roos, B. 0. Theor. Chim. Acta, 1995, 90, 87.
Nguyen et al. an active space of 10 molecular orbitals. This choice was based on the consideration that the active space should be large enough to include the most important nondynamical correlation effects for all structures considered. The population analysis of the natural orbitals resulting from the CASSCF calculations indicates that, in all cases, all orbitals with a population lying between 0.02 and 1.98 are included. In both MP2 and CASPT2 calculations, all valence electrons were correlated whereas core orbitals were kept frozen. Selected geometrical parameters are displayed in Figures 1 and 2 for [CZH~NP] and in Figure 6 for [C2H5NP]+. The corresponding total, zero-point and relative energies are recorded in Tables 1-3. As for the notation, A/B denotes a transition structure (TS) connecting both equilibrium structures A and B. The indentity of each TS is determined by intrinsic reaction coordinate (IRC) calculations. The schematic potential energy profiles for HNW CH2=PH showing the cycloadditions and rearrangements are illustrated in Figures 3 and 5 . Throughout this paper, total energies are given in hartrees, zero-point vibrational and relative energies in kilocalories per mole, bond lengths in angstroms and bond angles in degrees.
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Discussion Cycloadducts and Isomers. Formally, phosphiranimine (1) and both cyclic carbenes 4 and 5 are the [2+1] and [2+2] primary cycloadducts, respectively, of the addition of phosphaethene to hydrogen isocyanide. In the [2+2] cycloaddition two distinct approaches giving two regioisomers (4 and 5) are in fact possible. Phosphaziridine 2 and exocyclic phosphaalkene derivative 3 are the alternative structures of 1 through ringopening whereas the cyclobutene derivative 7 could be formed from 4 by a 1,Zhydrogen shift (Figure 1). As seen in the first half of Table 1, the different methods and basis sets used essentially lead to the same energy ordering for the equilibrium structures 1-7. The effect of going from the segmented 6-3 1lG(d,p) basis to the AN0 basis is in some cases significant (up to 3 kcavmol), as indicated by the MP2 results obtained with both basis sets. The differences between the relative energies obtained by MP2 and CASPT2 methods are smaller, less than 2 k c d m o l for all structures. This is an indication that nondynamical correlation effects are only of minor importance and reliable results on equilibrium structures can be obtained from single-reference HF-based methods. An additional confirmation of this fact comes from the weights w of the reference wave functions in the final wave functions which are similar for all structures already at the MP2 level. Unless otherwise noted, the relative energies quoted hereafter refer to the CASPT2 ZPE values. The syn form l a of phosphiranimine in which both atoms P and H(N) are disposed in a cis conformation with respect to the C=N bond is marginally more stable than anti-lb (0.8 k c d mol). A similar situation has previously been found for the sulfur and silicon analogue^.^-'^^'^ Both molecules 2a and 2b containing a phosphaziridine cyclic structure and an exocyclic C-C double bond have essentially a planar configuration of the four heavy atoms. The syn form 2a is about 1 kcaYmol less stable than anti-2b, in part due to the repulsion between the N and P lone pairs. The third alternative ring system 3 with an exocyclic C=P bond is calculated to have an almost planar heavy atom skeleton, but the nitrogen atom remains pyramidal.
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(12) Nguyen, M. T.; Vansweevelt, H.; Vanquickenbome, L. G. J . Org. Chem. 1991, 56, 5651. (13) Vanquickenbome, L. G.; Vansweevelt, H.; Coussens, B.; Hajnal, M. R.; Delvaux, I.; Hendrickx, M. J . Phys. Chem. 1990, 54, 1335.
J. Am. Chem. SOC., Vol. 117, No. 28, 1995 7537
Reaction of Phosphaethene with Hydrogen Isocyanide
lb
la
2b
2a
H\N-
1.338
8'R
ci
3
4 '
H
H\c 1.676
$'
+ 1.186
C=N-H
6
Figure 1. Selected MP2/6-31G(d,p) geometrical parameters of the [CzbNP] equilibrium structures considered.
Table 1. Total (au) and Zero-Point Vibrational (kcaVmol) Energies for the [C&NP] Stationary Points Considered
structurea la lb 2a 2b 3 4 5 6 7 ldla ldlb ld2b ld4 2d2b 4/2b 4/7 614 615 6/la(C) 6/la(P) 6/lb(P)
MP2/6-31lG(d,p) -473.789 18 -473.786 08 -473.776 05 -473.777 23 -473.767 17 -473.743 50 -473.733 75 -473.759 53 -473.792 96 -473.706 03 -473.743 12 -473.717 96 -473.701 44 -473.764 52 -473.701 58 -473.682 66 -473.674 68 473.654 63 -473.718 17 -473.714 17 -473.716 33
CISDQ/6-31 lG(d,p) -473.825 40 -473.822 74 -473.812 27 -473.813 41 -473.802 95 -473.786 99 -473.777 88 -473.802 27 -473.828 89 -473.738 29 -473.777 08 -473.747 02 -473.734 55 -473.800 16 -473.735 25 -473.719 58 -473.711 98 -473.691 51 -413.752 47 -473.747 64 -473.749 28
QCISD(T)/6-31G(d,p) -473.772 90 -473.769 87 -473.759 97 -473.762 01 -473.752 67 -473.730 22 -473.777 88 -473.747 69 -473.776 09 -473.713 38 -473.725 06 -473.713 28 -473.683 45 -473.746 62 -473.684 10 -473.663 57 -473.658 53 -473.637 60 -473.708 05 -473.698 44 -473.699 40
MP2/ANOb -473.805 37(0.81) -473.802 49(0.81) -473.795 64(0.81) -473.797 Ol(0.81) -473.781 79(0.81) -473.760 79(0.83) -473.751 40(0.82) -473.776 13(0.81) -473.812 87(0.82) -473.731 05(0.77) -473.762 52(0.81) -473.741 58(0.77) -473.722 69(0.81) -473.783 47(0.82) -473.724 92(0.81) -473.701 18(0.82) -473.695 19(0.81) -473.675 98(0.80) -473.738 31(0.80) -473.737 03(0.80) -473.739 31(0.80)
CASFT2/ANOb -473.810 78(0.85) -473.809 25(0.85) -473.798 51(0.85) -473.799 89(0.85) -473.786 19(0.85) -473.766 24(0.86) -473.758 0 l(0.85) -473.780 79(0.86) -473.8 16 76(0.85) -473.762 82(0.84) -473.766 32(0.85) -473.762 21(0.84) -473.726 54(0.84) -473.786 67(0.85) -473.726 15(0.85) -473.705 62(0.84) -473.698 92(0.85) -473.680 73(0.85) -473.750 12(0.85) -473.743 OS(0.85) -473.745 72(0.84)
ZPEC 34.5 34.3 34.2 34.1 34.8 34.6 35.0 29.3 35.1 33.1 32.7 32.9 33.9 33.5 33.8 32.0 31.6 32.3 31.7 31.6 31.6
Based on MP2/6-31G(d,p)optimized geometries given in Figures 1 and 2. All core orbitals are kept frozen in CI and MP2 calculations. See text for details of the AN0 basis set. The weights o of the zero-order wave functions in the final first-orderwave functions are given in parentheses. Zero-point vibrational energies, from the HF/6-3lG(d,p) calculations and scaled by 0.9.
As would be expected, the C-N stretching wavenumber of la, estimated to be centered at 1714 cm-I, tums out to be slightly smaller than the corresponding values of 1780 cm-l for cyclopropanimine, 1742 cm-' for thiiranimine, and 1754 cm-' for siliranimine: but remains much larger than that of 1640 cm-' for methanimine (CH2=NH).I2 A similar trend is observed for the C-C and C=P stretching wavenumbers in 2 and 3, respectively. The upward shift of the exocyclic double bond stretching wavenumber is a consequence of its coupling to a three-membered ring, irrespective of the nature of the ring atoms. l 2 The four-membered rings 4, 5, and 7 contain a quasi-planar geometry of the four heavy atoms. While the nitrogen atom has a planar configuration in the cycles, similar to that in aminocarbenes (with a short C-N distance of 1.333 A) and vinylamines, the phosphorus atom remains strongly pyramidal. Of the nine equilibrium structures considered, phosphaazacyclobutene (7)tums out to be the most stable isomer, lying about 3 kcallmol below la. The latter is the most stable among
the five possible three-membered rings, followed by the endocyclic phosphaziridines 2a and 2b and finally the exocyclic phosphaalkene derivative 3. While the fragments HN=C CH2=PH (6)have an energetic content comparable to that of 3, the cyclic carbene 5 is found to be the least stable isomer, being 34 k c d m o l higher in energy than la. The other cyclic carbene 4 is also a high-energy isomer lying 29 kcdmol above la. This is in clear contrast with the situation of the silicon analogues where the endocyclic silaziridine IV was calculated to be the most stable species and where siliranimine I11 and the cyclic carbene V both have similar relative energies (eq 1 and 2).6 Presumably repulsion between the lone pairs of the C, N, and P atoms is responsible for a certain destabilization of the structures 2, 4, and 5. It is beyond doubt that the lower stability of 3 arises from the presence of the weaker exocylic C=P bond, relative to the C=C and C=N bonds. Cycloadditions and Interconversions between Cycloadducts. Energies of the transition structures (TS) (Figure 2) for various processes are given in the second half of Table 1. The
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7538 J. Am. Chem. SOC., Vol. 117, No. 28, I995
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Nguyen et al.
H/r
8'
H/p
l d l a (C,)
tRC1NCzF199.9
ld2b ldlb
n
614 yNc;c;P)I71.4
2al2b
417
H\ HI"'
IH
1.737 82.4
1.207 -PH+NH3+CH4
NH
-‘C=W
+
(0
%NHZ
-33.6 kcsVmol
NH/
Figure 7. Some isodesmic reactions for 3 and 3H. The values are the heats of reaction calculated at the HF/6-31G(d,p) ZPE level.
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follows. The least exothermic reactions are (a) in the protonated system and (d) in the neutral system. Because in these equations the products do not have an amino substituent on the HC=PH2skeleton, the smaller exothermicity is for the greater part due to a resonance stabilization in the ring system. The smaller heat of reaction for the protonated system (a) compared to the neutral (d) arises from the larger ring resonance ability of 3H compared to 3. Reactions b and c and their counterparts e and f describe the influence of the resonance of the amino function on the exocyclic C=PH(H+) moiety. Indeed the larger exothermicity of these reactions is given by the resonance factor present on the right hand side of the equations. A combination of an alkyl group and an amino group stabilizes the C-PH(H+) entities, and the reactions c and f are therefore more exothermic. A comparison of the heats of the reactions (a-c) with their neutral counterparts d-f suggests that the protonated rings are much more stabilized through resonance. The difference between the heats of reactions c and e, for example, can-to a large extent-be traced back to the stabilization of 3H relative to 3. Nevertheless, the most stable protonated forms turns out to be the open structure 8H which is a phosphino derivative of the methylnitrilium cation. 8H is 32.3 and 38.4 kcdmol lower in energy than 1H and the reactants HNS2 H2C=PH2+ (6H), respectively. Regarding the [2+ 13 cycloaddition, we have examined the attack on both C and P centers. Qualitatively,
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Conclusions A number of results emerge from the present theoretical study on the cycloaddition of the model H N l C CH2=PH and HNGC H2C=PH2+ systems. (1) The [2+1] cycloaddition is the most favored mode of the HN=C CH2=PH reaction. This gives rise to phosphiranimine products l a and l b in which the reaction path via a C-attack leading to l a is slightly preferred by a stereoelectronic effect. Our best estimate predicts a classical barrier height of 35 kcdmol. (2) The most stable isomer among the possible threemembered rings resulting from the J5NCH2=PH reaction is the phosphiranimine la. Structure 2 with an exocyclic C=C
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J. Am. Chem. SOC., Vol. 117, No. 28, 1995 1543
Reaction of Phosphaethene with Hydrogen Isocyanide bond could be formed as a minor product through isomerization of 1; the TS ld2b lies in fact lower in energy than the TS 6/la(C) for [2+1] cycloaddition. Structure 3 with an exocyclic C-P bond is thermodynamically less stable (16 kcdmol above
la). (3) As far as the four-membered rings 4, 5, and 7 are concemed, the latter is the most stable of all isomers considered while the former are the least stable isomers. Although the carbene ring 4 is in principle involved in the [2+2] cycloaddition, its low thermodynamical stability caused by electron pair repulsion renders the [2+2] cycloaddition hardly competitive with respect to the [2+ 11reaction. However being surrounded
by high potential wells, the cyclic carbene 4 might exist as a discrete species if it could be generated by other means. (4)The effect of protonation at phosphorus results in an overall stabilization of the ring structures, but the most favored reaction mode of HNW H*C=PH*+ is a nucleophilic addition giving an open phosphino derivative of the methylnitrilium cation.
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Acknowledgment. We are indebted to the Science Organisations of Belgium NFWO, IWT, and DPWB-DWTC for continuing financial support. JA950604G
